Since liquid crystal panels have a low driving voltage and low power consumption and can easily performs multiple gray-scale display, the liquid crystal panels are widely used in large-scaled displays.
The liquid crystal panels visually display image information using external light, because the liquid crystal panels do not emit light voluntarily. The liquid crystal display apparatuses are classified into a reflective type and a transmissive type depending on mechanisms lighting the liquid crystal panels. Since the reflective liquid crystal display apparatuses use external light, the reflective liquid crystal display apparatuses cannot visually display image information in dark places such as outdoors at night and dark indoors. In order to visually display the image information in the dark places, the transmissive liquid crystal display apparatuses include an illumination device called a backlight lighting the liquid crystal panels from a rear side thereof.
A cold cathode fluorescent lamp (abbreviated as CCFL) is widely used in the backlight. The cold cathode fluorescent lamp has a large size, thereby making it difficult to reduce the size of the backlight. In addition, since the cold cathode fluorescent lamp requires a booster circuit and a stabilizer for a driving circuit, there is a problem in that the configuration of the backlight is complicated. In order to solve such a problem, with the practical utilization of a blue light emitting diode (abbreviated as “LED”), there has been suggested an illumination device employing an LED that can be easily reduced in size in comparison with the cold cathode fluorescent lamp and that does not require a booster circuit and a stabilizer for a driving circuit as a backlight.
In the related art, an illumination device including plural LEDs has been known (for example, see Japanese Unexamined Patent Publication JP-A 2003-162229).
In addition, in another related art, an illumination device controlling brightness on the basis of character information, image information, and photograph information displayed on the liquid crystal panel has been known (for example, Japanese Unexamined Patent Publication JP-A 2005-49631).
Since an LED has smaller light intensity than the CCFL, most illumination devices include plural LEDs. Particularly, since an illumination device lighting a large-sized liquid crystal panel should light a large-area surface, the illumination device has a large size and requires several hundreds of LEDs.
The forward voltage drop of the respective LEDs is in the range of 1.5 V to 4.5 V. The forward voltage drop of the respective LEDs hardly varies with a variation in current and has a steady voltage characteristic. The light intensity emitted from the respective LEDs is controlled by adjusting the flowing current and current of 10 mA to 100 mA usually flows in the respective LEDs.
A illumination device having a configuration in which plural LEDs are connected in parallel and an illumination device having a configuration in which plural LEDs are connected in series have merits and defects. When the plural LEDs are connected in parallel, the voltage to be supplied from a power source corresponds to the forward voltage drop of one LED and is in the range of 1.5 V to 4.5 V, but the current to be supplied from the power source has a magnitude obtained by adding the current flowing in the LEDs. When the current to be supplied from the power source increases, power is consumed by wires from the power source to the LED, thereby increasing the resistance loss. For example, when 500 LEDs are connected in parallel and the current flowing in each LED is 20 mA, the current to be supplied from the power source is 10 A (500×20 mA) and thus the resistance loss increases.
When the plural LEDs are connected in series, the current to be supplied from the power source corresponds to the current flowing in one LED and is in the range of 10 mA to 100 mA, but the voltage to be supplied from the power source has a magnitude by adding the forward voltage drops of the LEDs. In this case, a high-voltage power source is necessary and thus the device is complicated. For example, when 500 LEDs are connected in parallel and the forward voltage drop of each LED is 4 V, the voltage to be supplied from the power source is 2 kV (500×4 V).
On the basis of the balance between the magnitude of the resistance loss and the complication of the power source, a general illumination device has a configuration in which plural LED arrays having LEDs connected in series are connected in parallel. In this case, when one LED is out of order and enters an open mode, that is, a disconnected state in which no current flows, the LED array including the out-of-order LED enters the open mode and thus all the LEDs in the LED array are extinguished.
FIG. 11 is a circuit diagram schematically illustrating a part of an illumination device 1 according to another related art. The illumination device 1 has a configuration in which two LEDs 2 and 3 are connected in parallel and LED units 4 each composed of two LEDs 2 and 3 connected in parallel are connected in series. When one LED 2 of the LED unit 4 is out of order and enters the open mode, but the other LED 3 is in a connected state, the LED unit 4 does not enter the open mode, thereby preventing all the LEDs 2 and 3 of the LED array from being extinguished.
When the mean time before failure of the LEDs is 106 hours, the illumination device having several hundreds of LEDs has the possibility that the LEDs are out of order at the rate of one to several thousands of hours. In the conventional illumination device 1 shown in FIG. 11, when one LED 2 of the LED unit is out of order and enters the open mode, the current having flown in the out-of-order LED 2 flows in the other LED 3. Accordingly, the current flowing in the other LED 3 increases and thus the load of the other LED 3 increases. In the conventional illumination device 1, since the voltages applied to the LEDs 2 and 3 cannot be measured individually, it is difficult to detect the disorder of the LEDs 2 and 3 and it is difficult to establish the countermeasure to the disorder of the LEDs 2 and 3.
When plural LED arrays each having LEDs connected in series are connected in parallel and, as described above, one LED is out of order into a disconnected state, there is a problem in that all the LEDs of the LED array including the out-of-order LED are extinguished.